The present invention generally relates to a method for end point detection in chemical mechanical polishing and more particularly, relates to a dual detection method for end point in chemical mechanical polishing by utilizing both an optical method and an acoustical method.
In the fabrication of a semiconductor devices, such as silicon wafers, a variety of different semiconductor equipment and/or processing tools are utilized. One of those processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in modern memory devices. The planarization process is important since in order to fabricate the next level circuit, a high resolution lithographic process must be utilized. The accuracy of a high resolution lithographic process can only be obtained when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of a semiconductor device.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modem semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile into the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not co-linear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed informally between about 5000 xc3x85 and about 10,000 xc3x85. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also be applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handle are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrolyzation reaction; the formation of hydrogen bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in end point detection. The CMP process is frequently carried out without a clear signal about when the process is completed by using only empirical polishing rates and timed polish. Since the calculation of polish time required based on empirical polishing rates is frequently inaccurate, the empirical method fails frequently resulting in serious yield drops. Attempts have been made to utilize in end point mechanism including those of capacitive measurements and optical measurements. However, none of these techniques have been proven to be satisfactory in achieving accurate control of the dielectric layer removed.
Another method for achieving end point detection is marketed by the Applied Materials Corporation of Santa Clara, Calif. In a MIRRA(copyright) CMP device. In the MIRRA(copyright) device, a system of in-situ remote monitor (ISRM) is provided to determine end point by the concept of a periodic change of optical interference. In the MIRRA(copyright) device, signals received from a patterned wafer surface are processed by digital filtering algorithms by a PC programmable filter such that an optical interference intensity changes periodically with the thicknesses of removed surface material. For instance, a built-in laser source which is fixed at 6700 xc3x85 wavelength is utilized to cause interference at a wafer surface and thus producing a waveform received by a laser detector. The waveform generated by such a technique is shown in FIG. 1. Similarly, a reflectance vs time curve obtained in a LAM(copyright) CMP apparatus is shown in FIG. 4.
FIG. 1 illustrates four cycles of a waveform with each cycle corresponds to a removed material layer thickness of approximately 2437 xc3x85. The technique is adequate to detect an end point in a polishing process wherein only a relatively thin layer, for instance, of only 2000 xc3x85 is removed. When a large thickness of material such as an IMD oxide layer having a thickness of at least 4000 xc3x85 is to be removed, the method frequently produces faulty results since the laser detector cannot distinguish which one of the waveform cycles the end point falls on. The wafer surface can therefore be either over-polished or under-polished by 2400 xc3x85 thickness. For instance, FIG. 5A shows an over-polish condition of a present layer wherein Cu residue causes a short across metal lines, while FIG. 5B shows an over-polish of a previous layer under a present layer which leads to high resistance due to dishing/erosion, and furthermore, hard-to-remove Cu residue is left on the next layer.
U.S. Pat. No. 6,071,177, assigned to the common assignee of the present invention discloses a method and apparatus for determining end point in a chemical mechanical polishing process by utilizing a dual wavelength interference technique. The patent further discloses a method and apparatus for terminating a chemical mechanical polishing process conducted on a semiconductor wafer at a preset end point by utilizing a dual wavelength interference technique wherein the period, or a process window for detection is greatly expanded from that conventionally available of a single wavelength interference technique. For instance, instead of a process window of only 2400 xc3x85 (or a period between cycles in the waveform), the technique provides a window for detection of at least 4000 xc3x85 such that a greatly expanded detection range is provided. The method and apparatus may be utilized for detecting thickness of material removed from the surface of a semiconductor wafer of any thickness, but is particularly suitable for determining the thickness of material removed from a thick layer on a wafer surface.
In the method, a laser generating source which may include two semiconductor diodes each generating a laser emission at a different wavelength is utilized. For instance, a suitable set of laser emission wavelengths utilized is 4500 xc3x85 and 6700 xc3x85, respectively. While any suitable sets of wavelength of laser emission may be utilized, it is desirable that an two different wavelength emissions produce an additive effect in the waveform obtained after an interference process between emissions reflected from the wafer surface. For instance, instead of a period or a process window of 2400 xc3x85 available from a single wavelength interference pattern obtainable conventional, the present invention dual wavelength interference technique may produce a period of at least 4000 xc3x85, i.e., a window that is almost twice as wide as that available from the conventional single wavelengths interference technique. This provides the benefit of easy identification of an end point when a large thickness of dielectric material is removed.
The method provides an end point detection method for oxide CMP process which may be utilized in a planarization process for ILD, IMD or STI layers by the concept of periodic interference change. By using a method of a dual wavelength interference, the range or the process window for end point detection can be greatly expanded, i.e., by almost 100%. The present invention end point detection method therefore increases wafer throughout efficiency and achieves wafer cost reduction.
Based on the concept of periodic change of optical interferences, the method for end point detection utilizes a dual frequency laser emission source positioned in a rotating platform for a CMP process. Signals obtained from patterned wafer surface are processed through digital filtering algorithms in a PC programmable filter such that the interference intensity periodically changes with the thicknesses of the removed surface layer. For a built-in laser source having two semiconductor diodes at 4500 xc3x85 and 6700 xc3x85 wavelengths respectively, each cycle obtained in a composite waveform corresponds to a removed thickness of approximately 4000 xc3x85. The present invention novel method therefore can be used to cover a wide range of dielectric layers including those of ILD, STI and IMD process windows.
U.S. Pat. No. 6,071,177 further shows, in FIG. 2, an apparatus 10 which has a polishing platen 12 that is intimately joined to a polishing pad 14 and is used as the rotating platen in the CMD apparatus. The rotating platen 12 is equipped with a laser generating source 20 which includes two semiconductor diodes (not shown) each capable of generating laser emissions at a different wavelength. For instance, as shown in FIG. 2, laser emissions 22 and 24 are each generated by a semiconductor diode at 4500 xc3x85 and 6700 xc3x85, respectively.
Also shown in FIG. 2 is a semiconductor device 16 which consists of an oxide coating layer 18 overlying a base material layer 28. The base material layer 28 may be formed of any suitable materials, including but not limited to silicon, polysilicon and metal. The semiconductor water 16 is pressed onto the rotating platen 12 such that a top surface 32 of the oxide layer 18 intimately contacting and frictionally engaging a top surface 34 of the polishing pad 14. Laser emissions 22, 24 from the laser source 20 irradiating onto surface 32 of the oxide layer 18 through a window 40 provided in the polishing pad 14. This is also shown in a plane view of FIG. 3.
The laser emissions 22, 24 from the laser source 20 are partially reflected by the oxide surface 32 into reflected beams 36 and 38. Part of the laser beams 22, 24 penetrates into the oxide layer 18 and are reflected by the interface 50 formed between the oxide layer 18 and the base material layer 28. The reflected beams are then deflected at the oxide surface 32 into laser beams 56 and 58 to be received by the laser detector 30. An interference occurs between the deflected beams 36, 38 and the deflected beams 56, 58 when the beams are received by the detector 30 to thus producing a dual wavelength interference waveform 60 (shown in FIG. 4).
While the method provided in U.S. Pat. No. 6,071,177 works somewhat satisfactorily, there are several drawbacks in the conventional single wavelength interference method. For instance, the technique requires complicated equipment set up, i.e., dual laser sources and receivers, and furthermore, requires a high degree of training for the operator to operate the system. Moreover, when the window 40 shown in FIG. 2 is damaged, either broken or fractured, or simply covered with a slurry material, the efficiency of detection suffers greatly.
It is therefore an object of the present invention to provide a method for detecting an end point in a chemical mechanical polishing process that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for detecting an end point in a chemical mechanical polishing process based on an acoustical detection principle.
It is a further object of the present invention to provide a method for detecting an end point in a chemical mechanical polishing process that does not rely solely on optical detection.
It is another further object of the present invention to provide a dual detection method for end point in chemical mechanical polishing wherein both an optical detection device and an acoustical detection device are utilized.
It is still another object of the present invention to provide a dual detection method for end point in chemical mechanical polishing that utilizes an acoustical sensing device such as a microphone.
It is yet another object of the present invention to provide a chemical mechanical polishing apparatus that is equipped an acoustical end point detection device without using an optical detection device.
It is still another further object of the present invention to provide a method for detecting an end point in a chemical mechanical polishing process by detecting a volume of the polishing acoustical signals and determining the end point when the volume decreases by at least 30% of its initial volume.
In accordance with the present invention, a dual detection method for end point in chemical mechanical polishing by utilizing both an optical detection device and an acoustical detection device is provided.
In a preferred embodiment, a method for detecting an end point in a chemical mechanical polishing process can be carried out by the operating steps of providing a CMP apparatus contained in an enclosure; mounting an acoustical sensor in the enclosure; initiating a CMP process on a semiconductor wafer for removing an uppermost coating layer; monitoring an acoustical signal generated by the CMP process and recording a volume of the signal; and stopping the CMP process when the volume of the acoustical signal changes by at least 30% of its initial volume.
The method for detecting an end point in a CMP process may further include the step of stopping the CMP process when the volume of the acoustical signal changes by at least 50% of its initial volume, or the step of stopping the CMP process when the volume of the acoustical signal decreases by at least 50% of its initial volume. The method may further include the step of stopping the CMP process when the volume of the acoustical signal decreases by at least 5 dB from its initial volume. The method may further include the step of mounting a microphone in the enclosure.
The present invention is further directed to a CMP apparatus that is equipped with an acoustical end point detection device which includes a polishing pad holder for holding a polishing pad thereon; means for rotating the polishing pad holder; a sample holder for holding a sample to be polished thereon; means for rotating and traversing the sample holder on the polishing platen; an enclosure for enclosing the polishing pad holder and the sample holder; and an acoustical sensor mounted in the enclosure for sensing acoustically an end point of the polishing process when a change in volume of at least 30% occurs.
In the CMP apparatus that is equipped with an acoustical end point detection device, the acoustical sensor may be a microphone. The acoustical sensor senses an end point of the CMP process when a change in volume of at least 50% occurs, or when a decrease in volume of at least 50% occurs. The acoustical sensor further senses an end point of the CMP process when at least a 5 dB drop in volume occurs. The CMP apparatus may be of the rotary type, or may be of the linear type.
The present invention is still further directed to a dual detection method for an end point in a chemical mechanical polishing process which can be carried out by the operating steps of providing a CMP apparatus that is contained in an enclosure, the CMP apparatus includes a polishing pad holder equipped with an optical projector and a sensor therein for projecting and receiving an optical signal to and from a surface of a sample that is being polished through a window in the polishing pad for determining an end point of the CMP process; mounting an acoustical sensor in the enclosure for sensing and recording an acoustical signal generated by the CMP process; initiating a CMP process on the sample for removing an uppermost coating layer; determining an end point of the CMP process upon the occurrence of at least one of the two events; the optical signal received from the sample surface being indicative of an interface between the uppermost coating layer and an underlying layer, and the acoustical signal generated by the CMP process changes by at least 30% of its initial value.
The dual detection method for an end point in a CMP process may further include the step of determining the end point of the CMP process when the acoustic signal produced by the CMP process changes by at least 50% of its initial value, or the step of determining the end point of the CMP process when the acoustical signal produced by the CMP process decreases by at least 50% of its initial value. The method may further include the step of determining the end point of the CMP process when the acoustical signal produced by the CMP process decreases by at least 5 dB from its initial value. The method may further include the step of projecting and receiving a laser beam to and from a surface of the sample, or the step of mounting an acoustical microphone in the enclosure. The CMP apparatus may either be a rotary type CMP, or a linear type CMP.